Our invention relates to methods of imaging recurring high speed events, and is a variation of the method disclosed in U.S. Pat. No. 4,970,597 to Shepard for high speed imaging with a standard video camera. Of interest also is U.S. Pat. No. 5,136,383 to Shepard and Sass, which describes hardware useful in the technique of the '597 patent. We believe that U.S. Pat. Nos. 4,970,597 and 5,136,383 and references cited therein are the most relevant prior art to the present invention.
In the Shepard '597 method, the camera executes raster patterns wherein the camera repeatedly scans a series of parallel horizontal lines which comprise an image field. The camera then generates a continuous video signal comprised of line signals representing the horizontal lines. As the camera continuously views the recurring event, a flagging signal is generated before each recurrence of the event. Within a complete image field, one or more of the recurring events may begin during an aperture signal, and the line then being scanned will accurately represent a portion of the event. Signals or data representing these accurate, or synchronized, lines are collected and stored in a computer memory.
The Shepard '597 method uses a commercially available infrared imaging camera having RS-170 video output. The raster pattern of the camera is traced by two periodically driven scanning mirrors, one driven vertically and one driven horizontally. The vertical scanning mirror is driven at a frequency of 60 Hz and the horizontal scanning mirror is driven at a frequency of 4 kHz. The vertical scanning mirror frequency is appropriate for generating a video signal, but the horizontal mirror frequency is one-fourth as fast as needed for RS-170 video output.
Two camera features remedy the horizontal mirror's slowness. First, the camera accepts input from the horizontal mirror during both forward and reverse passes and digitizes the data from these passes. Since the camera receives "reverse scan" data in reverse order, the camera re-reverses this data into proper, "forward scan" order. Use of data from reverse passes speeds the effective frequency of the horizontal mirror but impairs image accuracy when rapidly changing events are viewed.
The second camera feature increasing the horizontal mirror's effective frequency is the duplicated output of each scanned line. Thus, if the first horizontal line of the camera's raster pattern output is a forward scanned line, then the second line of the camera's raster pattern output is a copy of the first forward scan line. The third line of the camera's raster pattern output will be a reverse scanned output and the fourth output line will be a copy of the third, reverse scanned line. The raster pattern output from the camera will be repetitions of the sequence just described. As described in Shepard '597 and Shepard/Sass '383, one can limit which camera raster output lines will be used to compile a composite image of a given event. No matter what output lines are used by the method in these references, the camera's effective frame rate camera was limited only by the 125 microseconds required for a single scan line, whereby the effective frame rate of the camera was increased to 4,000 frames per second.